The present invention relates to an apparatus for screening capsules during the manufacturing and filling process wherein the capsules are screened and classified at a relatively high production rate. The invention relates to U.S. Pat. No. 4,223,751, issued Sep. 23, 1980, and entitled "High-Speed Capacitance Apparatus for Classifying Pharmaceutical Capsules." The present invention comprises an enhancement and improvement to the machine disclosed in the aforementioned patent which enables a higher degree of accuracy in the operation of the aforementioned machine.
The machine described in the aforementioned patent includes a hopper for holding a supply a capsules of a type and size commonly used in the pharmaceutical industry. The capsules in the hopper are conveyed to a turntable device which has a chute tangentially positioned along its circumference. Capsules are fed into the chute in a continuous stream, and the chute conveys the capsules into an air jet mechanism which propels them through a capacitive sensing device. The capacitance sensing device detects variations in weight of the capsules by virtue of their respective capacitance measurements as the capsules are propelled through the sensing device. A mechanical deflector may be energized to deflect capsules which fall outside of predetermined weight tolerances into a different trajectory than the trajectory of the "good" capsules. The respective trajectories lead to collection receptacles wherein the "good" and "bad" capsules may be separately collected. The machine is designed to operate at fairly high rates of speed and is capable of processing and weighing capsules at rates in excess of 2,500 capsules per minute.
While the aforementioned machine is capable of operating at the stated processing speed, there are some problems presented by particular capsules or batches of capsules which can lead to systemic errors in operating the machine. The basic technology utilized by the machine in providing its weight measurements and thereby classifying capsules by weight is a capacitance sensor technology. This technology takes advantage of the fact that the capacitance measurement of a capsule varies linearly with the mass of the capsule; and therefore, capacitance measurements made of a stream of capsules propelled through the sensor provide a stream of weight measurements corresponding to the capsule's weight. The technology assumes that the capsules are of the same size and shape and have a uniform dielectric constant; and under these assumptions, the apparatus functions very well.
Non-uniformity of the dielectrics in a batch of capsules can occur as a result of general environmental effects and also as a result of random differences in the dielectric constant of the capsule material. The environmental changes are first order effects in particular caused by relative humidity or water vapor; the variation in dielectric constant caused by material change is controllable during earlier stages of the manufacturing process and is, therefore, relatively rare.
The general environmental effects on a capsule are quite understandable when an analysis of the material and function of the capsule is understood. The outer shell of a capsule is made of a solid state gelatin material which is supposed to dissolve or melt in the body and, therefore, is very hygroscopic. The gelatin material is designed to start swelling and dissolving at approximately 60%-70% relative humidity (RH).
Previous research conducted by the assignee of the present invention has compared capsules which have been weighed gravimetrically and also have been weighed utilizing the capacitance measuring techniques at different RH equilibriums. These research results show that the capacitance measurement of a capsule generally increases linearly as the RH increases from 20%-60% RH. The perceived weight predicted by the capacitance measurement increases about 30% over this range of RH variation. As the capsule RH is increased above about 60%, the perceived weight measurement rather drastically increases, probably caused as much by the fact that the capsule is beginning to dissolve as any other factor. This research shows that the aforementioned apparatus should not be used as a capsule screening and weighing machine when the capsule and/or ambient RH is greater than about 60%. When the capsule and/or ambient RH is below about 60%, a simple calibration procedure can be used to correct the capacitance-based measurement to correspond to the actual gravimetric measurement. Since the capsules are typically stored under ambient conditions, the correction can be made simply by measuring the ambient RH in the vicinity of the machine during operating periods. In typical operation, this problem is adequately resolved if the calibration procedure is performed approximately once a day while the machine is being used.
The affects of random differences in dielectric constant are seldom noticed in capsules manufactured as a part of a single batch; the change most often occurs when comparing capsules made in different batches. Occasionally a capsule batch will contain some capsules that have a different dielectric constant, and this is usually caused by storage conditions or presorting conditions wherein different drums of capsules may be combined and mixed together, or where a partial drum of capsules produced in one batch is refilled from a drum of capsules made from a second batch. While such effects are relatively rare, it is now possible to take these effects into account by reason of the present invention.
Another cause of moisture variations between capsules in the same batch can arise if the user stores the capsules in a drum containing a small water adsorbing bag. In this case, the capsules closest to the bag will contain less water than other capsules in the drum and will therefore have a lower dielectric coefficient than the other capsules, thereby causing a lower reading of capacitance, and leading to an apparent reduction in weight when measured by the machine utilizing capacitance measurements for weight determination.
While it has been observed that all capsules which are propelled through the capacitance measuring device are propelled with the same initial force, the heavier capsules will reach a lower average velocity and the lighter capsules will reach a higher average velocity. Since the velocity of a capsule can be calculated by measuring the time the capsule requires to pass between two known points, the mass (weight) of the capsule can be readily calculated based upon these time measurements. The present invention, therefore, utilizes time measurements to provide a measure of velocity and, therefore, a measure of weight which is used as an enhancement and improvement over the apparatus described in the aforementioned prior art patent. The velocity measurement is used as a supplementary measurement, in addition to the capacitance measurement, in order to have two checks of the capsule weight to improve overall reliability of the machine to reject all capsules which may be heavier or lighter than the published specification for capsule weights. Because each technique has some accuracy range, the use of the two techniques together ensures that capsules will be rejected if either measurement test fails even though one or the other test may actually present a false rejection measurement because of its own accuracy limitations. According to industry standards it is better to reject an occasional capsule on the basis of an inaccurate measurement than to accept an occasional capsule which is outside permissible weight specifications.